Are there different filler tubes as well? Thatmight be my problem as well. The car had a fuel cell and the PO put in a junkyard tank fuel filler tube etc but didn;t have the vent tube to go with it.
yes there are 2 different filler necks. this might have been your problem. the original 71 fillerneck for a california emmisions car w/ the extra tube in the trunk is going to have a different cap. this cap is only going to fit the neck for that cap properly. a non emmision car will have a neck with a regular cap. the regular cap will be very loose on a emmision neck and leak. you would have to find the original cap for that style neck to make it fit properly.
